Electroless copper plating baths are in widespread use in metallization industries for depositing copper on various types of substrates. In the manufacture of printed circuit boards, for example, the electroless copper baths are used to deposit copper on walls of through-holes and circuit paths as a base for subsequent electrolytic copper plating. Electroless copper plating also is used in the decorative plastics industry for deposition of copper on non-conductive surfaces as a base for further plating of copper, nickel, gold, silver and other metals, as required. Electroless copper baths which are in commercial use today contain water soluble divalent copper compounds, chelating agents or complexing agents, for example, Rochelle salts and sodium salts of ethylenediamine tetraacetic acid, for the divalent copper ions, reducing agents, for example, formaldehyde, and formaldehyde precursors or derivatives, and various addition agents to make the bath more stable, adjust the plating rate and brighten the copper deposit.
It should be understood, however, that every component in the electroless copper bath has an effect on plating potential, and therefore, must be regulated in concentration to maintain the most desirable plating potential for particular ingredients and conditions of operation. Other factors which affect internal plating voltage, deposition quality and rate include temperature, degree of agitation, type and concentration of basic ingredients mentioned above.
In electroless copper plating baths, the components are continuously consumed such that the baths are in a constant state of change, thus consumed components must be periodically replenished. Control of the baths to maintain high plating rates with substantially uniform copper deposits over long periods of time is exceedingly difficult. Consumption and replenishment of bath components over several metal turnovers (MTO) can also contribute to bath instability, for example, through the buildup of side products. Therefore, such baths, and particularly those having a high plating potential, i.e. highly active baths, tend to become unstable and to spontaneously decompose with use. Such electroless copper bath instability can result in non-uniform or discontinuous copper plating along a surface. For example, in the manufacture of printed circuit boards, it is important to plate electroless copper on the walls of through-holes such that the copper deposit on the walls is substantially continuous and uniform with minimal, preferably, no break or gaps in the copper deposit. Such discontinuity of the copper deposit can ultimately lead to mal-functioning of any electrical device in which the defective printed circuit board is included.
Another issue associated with electroless copper plating is the stability of the electroless copper plating bath in the presence of high catalyst metal leaching. Electroless copper plating utilizes various metal containing catalysts, such as colloidal palladium-tin catalysts and ionic metal catalysts, to initiate the electroless copper plating process. Such metal containing catalysts can be sensitive to the plating conditions such as pH of the electroless copper bath, electroless plating temperature, components and concentrations of the components in the electroless copper baths, wherein such parameters can result in at least metal leaching from the catalyst, thus further destabilizing the electroless copper bath.
To address the foregoing stability issues, various chemical compounds categorized under the label “stabilizers” have been introduced to electroless copper plating baths. Examples of stabilizers which have been used in electroless copper plating baths are sulfur containing compounds, such as disulfides and thiols. However, many stabilizers lower electroless copper plating rates, and, also, at high concentrations can be catalyst poisons, thus reducing plating rates or inhibiting plating and compromising the performance of the plating bath. Low plating rates are detrimental to electroless copper plating performance. Electroless copper plating rate is also temperature dependent, thus when high stabilizer concentrations lower the rate, increasing the plating temperature can increase the rate. However, increasing the operating temperatures can decrease the stability of the electroless copper bath by increasing the buildup of byproducts as well as reducing bath additives by side reactions, thus negating some of the effects of increasing the stabilizer concentration. As a result, in most cases the amount of stabilizer used must be a careful compromise between maintaining a high plating rate and achieving an electroless bath that is stable over a long period of time. Rate acceleration in electroless copper plating is a key strategy for lowering working temperatures, lowering internal stress of copper deposits such as on flexible substrates and decreasing overall running costs of metallization.
Examples of flexible substrates are polyimides and polyimide matrix composites. Such polyimides and polyimide matrix composites are used in electronics, automotive, aerospace and other applications. Under conditions in which polyimides have absorbed water, either by exposure to high humidity or by direct immersion the electroless copper deposit on the polyimides may blister. Blister formation seriously compromises smooth and uniform copper layer coverage on polyimides. To avoid blistering, plating on the surfaces of polyimides requires the deposition of a low stress electroless copper deposit. Accordingly, stress reducers are typically included in such electroless copper baths. One commonly used stress reducer is 2,2′-bipyridyl which can reduce blistering on polyimide substrates. However, 2,2′-dibipyridyl is also a plating rate suppressor. To compensate for the rate suppressing effect of the 2,2′-dipyridyl, the temperature of the plating bath must be increased, thus increasing the probability of undesired blister formation resulting in irregular, dull and rough copper deposits and negating the purpose of including stress reducing additives such as 2,2′-dipyridyl in the electroless plating bath.
Therefore, there is a need for an additive for electroless copper plating baths which enables a high rate of electroless copper plating at low temperatures to provide bright, smooth and uniform copper deposits on substrates and prevents blistering of polyimides.